A device of the type mentioned above is described in the non-prepublished Netherlands Patent Application No. 860027, corresponding to U.S. Pat. No. 4,807,967.
In this document a problem with electroscopic fluid displays is described, which consists in that during operation electric charge accumulates in or on the insulating layers due to absorption of ions, the amount of absorbed ions increasing in time, also in the case of alternating voltage drive.
The above document also provides a solution for this charge-accumulation problem, namely by using a bare, i.e. having no insulating surface layers, silver movable electrode and fixed electrodes to which a polyimide layer is applied. It has been found, however, that in practice this solution is difficult to implement in particular as regards the lower substrate, since the technology required for the manufacture of an assembly of a lower substrate and movable electrodes annihilates the property of the polyimide that ions formed at the interface between the movable electrode and the non-transparent liquid are not absorbed at the interface.
It is an object of the invention to provide a workable solution to the known charge-accumulation problem.
This object is achieved by a device as described in the opening paragraph, characterized in that the degree of asymmetry of the alternating voltage drive is adapted to the difference in surface properties as regards charge delivery and charge adsorption of opposing insulating layers, or in that the alternating voltage drive is symmetrical, and opposing insulating layers have substantially the same surface properties as regards charge delivery and charge absorption.
For example, in a strongly injecting insulating layer the driving period can be relatively short, whereas in the case of a small absorbing opposing insulating layer the driving period can be relatively long. In practice, a symmetrical alternating voltage drive is probably to be preferred.
It is to be noted, that in the above document a description is given of an embodiment in which opposing insulating layers are made of the same material, i.e. silicon oxide, so that the insulating layers may have the same surface properties as regards charge delivery and charge adsorption. However, as is described in the document, the silicon oxide layers are applied to the main surfaces of the movable electrode, which consists of aluminum, to enhance the brightness of the picture to be displayed by the electroscopic fluid display and to provide an additional measure against short circuits between the movable electrode and the fixed electrode. In this connection, reference is made to the prepublished Netherlands Patent Application No.84 03 536, in which a description is given of an identical structure having opposing insulating layers consisting of silicon oxide, the layers each being provided with a monolayer of a silane compound which prevents charge adsorption by the respective insulating layer.
In accordance with the present invention, such monolayers of compounds containing, in general, polar and apolar groups are not necessary, while the combination of pairwise opposing identical insulating layers in combination with a pure alternating voltage drive is proposed for the first time as a possible measure to prevent charge accumulation.
An advantageous embodiment of the electroscopic fluid display is characterized according to the invention in that on at least one main surface of the movable electrode the insulating layer consists of anodized metal material of the movable electrode, and the insulating layer continues along the outer and inner peripheral portions of the perforated movable electrode, and in that the insulating layer on the substrate opposite the insulating layer of anodized metal material on the main surface of the movable electrode consists of an oxide of the same metal material.
This is also a solution in which opposing insulating layers are made of the same dielectric material, the dielectric material being obtained on at least one main surface of the movable electrode by anodizing the movable electrode, the apertures of the perforated movable walls determining the electrode and the side walls of the movable electrode also being provided with an insulating layer of anodized electrode metal, such that, as will be obvious, less charge carriers, such as ions, are injected into the contrast liquid in the electroscopic fluid display, which contributes to a reduction of the charge adsorption.
In a preferred embodiment, the movable aluminum electrode including, for example, circular apertures, is embedded in aluminum oxide obtained by anodizing the complete movable electrode, while aluminum oxide layers are applied to both substrates by, for example, sputtering.
Since the movable electrode is provided on at least one of its main surfaces with an insulating layer obtained by anodizing, an additional advantage can be obtained since in the case of a single anodic layer warpage of the movable electrode can be compensated or remedied by adjusting the thickness of the layer and, in the case of a movable aluminum electrode embedded in aluminum oxide the absence of warpage can be maintained.
The invention further relates to a method of manufacturing an electroscopic fluid display by providing a first structured electrode layer on a lower substrate, providing a first insulating layer on the lower substrate which is provided with the first structured electrode layer, providing a polymer layer on the first insulating layer, providing a second insulating layer on the polymer layer, providing a second structured electrode layer on the second insulating layer, selectively etching the second insulating layer using the second structured electrode layer as a mask, underetching the second insulating layer via the second structured electrode layer and, hence, selectively etching the polymer layer, providing an identically structured third insulating layer on the second structured electrode layer, the second structured electrode layer having such a pattern and the underetching being carried out such that a number of rotatable perforated electrodes is obtained which are interconnected by resilient connecting pieces which are supported by respective polymer supports, providing a fourth insulating layer on a transparent substrate and, finally, interconnecting the substrates in a tightly sealed manner, such that the third and the fourth insulating layer contact one another.
Such a method is known from the non-prepublished Netherlands Patent Application stated hereinbefore.
By means of this known method a movable electrode is obtained whose inner peripheral walls and side walls, which determine the apertures in the movable electrode, are not coated with an insulating layer, such that injection of the charge carrier into the contrast liquid may occur.
In accordance with the above stated object of the invention, it is an object to overcome this disadvantage also.